Airplane brake assemblies are well known in the art. FIG. 1 shows a typical prior art brake assembly 2. The brake assembly 2 includes a brake inlet port 4. The brake inlet port 4 is typically located at the top of the brake assembly 2 for purging air bubbles that may be present in brake assembly hydraulic fluid. The brake assembly 2 further includes a plurality of piston and cylinder assemblies 6, a pressure plate 8, a plurality of rotor and stator assemblies 10, and a torque tube 12. As can be seen in FIG. 1, the piston and cylinder assemblies 6 are located in a substantially circular arrangement about the brake assembly 2 for providing an even pressure distribution about the pressure plate 8.
FIG. 2 shows a schematic diagram of a typical brake assembly, such as the brake assembly 2 of FIG. 1. The brake inlet port 4 is shown at the top of the brake assembly 2. However, it is well known in the art to locate the inlet port at any position on the brake assembly. The brake inlet port 4 is coupled to a header from a hydraulic fluid reservoir (not shown). The cylinders 14, 16, 18, 19, 20, 22, and 24 are connected in series fluid communication via hydraulic manifold sections 26. The cylinders 14 and 24 are also coupled to the brake inlet port 4 via manifold sections 26. Flow restrictors 28 may be included in each of the manifold sections 26.
When a pilot applies an aircraft's brakes, hydraulic fluid flows from the hydraulic reservoir (not shown) through the header to the brake inlet port 4. When hydraulic fluid is introduced to the brake inlet port 4, hydraulic fluid first flows to the cylinders 14 and 24 that are coupled to the brake inlet port 4. Hydraulic fluid pressure increases within the cylinders 14 and 24 as they fill. When the pressure in the cylinders 14 and 24 reaches a predetermined threshold, their associated pistons actuate and press against the pressure plate 8, which clamps the rotor and stator assemblies 10 against the torque tube 12. After the cylinders 14 and 24 have filled, hydraulic fluid next flows through the manifold sections 26 to the cylinders 16 and 22. The cylinders 16 and 22 fill with hydraulic fluid and, in turn, hydraulic fluid flows through the manifold sections 26 to the cylinders 18 and 20. Finally, after flowing through the above-identified series of cylinders, hydraulic fluid flows to and fills the cylinder 19. Thus, the pistons associated with the cylinders 14, 16, 22, and 24, located toward the brake inlet 4 at the top of the brake assembly 2, actuate and clamp the rotor and stator assemblies 10 against the torque tube 12 before the pistons associated with the cylinders 18, 19, and 20, located toward the bottom of the brake assembly 2 away from the brake inlet port 4, actuate and clamp against the torque tube 12.
As a result, the rotor and stator assemblies 10 near the brake inlet port 4 at the top of the brake assembly 2 are compressed before the rotor and stator assemblies 10 farther away from the brake inlet port 4 at the bottom of the brake assembly 2. In some known brake assemblies, the time delay between actuation of piston and cylinder assemblies located closer to the brake inlet port and actuation of piston and cylinder assemblies located farther away from the brake inlet port has been recorded in excess of 50 ms. This time delay causes the torque tube 12 to react through the rotor and stator assemblies 10 at the top of the brake assembly 2 against the piston and cylinder assemblies 6 at the top of the brake assembly 2 while the piston and cylinder assemblies 6 at the bottom of the brake assembly 2 are still compressing against the torque tube 12. This results in a lateral displacement between the top and bottom regions of the rotor and stator assemblies 10 and the torque tube 12. When the piston and cylinder assemblies 6 at the bottom of the brake assembly 2 actuate, the bottom of the torque tube 12 reacts through the bottom region of the rotor and stator assemblies 10 against the piston and cylinder assemblies 6 at the top of the brake assembly 2. This, in combination with a return force from the top region of the torque tube 12 causes the clamping force at the top region of the brake assembly 2 to decrease relative to the clamping force at the bottom of the brake assembly 2. When the asymmetric clamping cycle and corresponding lateral displacement repeats for a rotating wheel, a vibration of the brake results. The vibration is known as "brake whirl" or "brake wheel vibration."
An imbalance in piston pressure exists across the brake assembly 2 during the time that the pistons are actuating. Piston pressures and torque tube clamping forces are not equalized across the brake assembly 2 during pressure transitions. Each piston actuates as an individual dynamic element within the brake assembly 2, rather than all the brake pistons actuating together as a single element. The resulting differential pressure between the pistons during braking undesirably reduces overall brake system stiffness.
The asymmetric clamping cycle described above can also cause stators to develop a tapered wear pattern. Further, the time delay between actuation of top and bottom region pistons degrades the frequency response of the brake assembly 2. Because the frequency response of the brake assembly 2 is lower than the frequency response of autobrake and antiskid valves included in aircraft brake systems, degradation in the brake assembly frequency response degrades frequency response of the overall braking system.
It would therefore be desirable to minimize time delays between actuation of brake pistons in order to minimize asymmetric clamping. Many prior art brake assemblies 2 include the flow restrictors 28 in the manifold sections 26 in an attempt to reduce brake wheel vibration or brake whirl. As can be appreciated, the flow restrictors 28 instead serve to further increase the time delay between actuation of brake pistons. Therefore, the flow restrictors 28 make brake clamping even more asymmetric and worsen the problems of brake whirl, uneven stator wear, and degraded frequency response. There is thus an unmet need in the art to minimize asymmetric brake clamping.